The invention relates to a device for the pneumatic conveyance of bulk material, with a conveying pipe which defines any desired conveying path for the bulk material, and with a propellant gas source which is connected to the conveying pipe and is designed for the supply of propellant gas in the direction of the conveying path, and to a corresponding method.
For the conveyance of granular or pulverulent material (bulk material), pneumatic conveying devices are known from prior public use. At a feed point, the bulk material to be conveyed is supplied to the conveying device, by which it is then transported by means of a gas stream in a conveying pipeline to a discharge point. This gas stream (propellant gas) may be generated by suction or by pressure; either a suction or a pneumatic pressure conveying device is referred to. These pneumatic conveying devices make it possible to transport the bulk material along virtually any desired path from the feed point to the discharge point. In this case, considerable height differences can be overcome, in particular the discharge point may even lie at higher levels than the feed point for the bulk material. Pneumatic conveying devices present difficulties particularly at those points where the bulk material to be conveyed occurs in a high concentration, for example at the feed point. So that this locally increased bulk material quantity can be absorbed into the propellant gas stream, the propellant gas must have a considerable minimum gas velocity. Providing the propellant gas stream with an appropriately high velocity is complicated, specifically both in structural terms, particularly with regard to the size of the pressure generator and filter required, and from cost standpoints, particularly with regard to the energy outlay required.
There is another known type of pneumatic conveying devices which manage with relatively low gas streams and consequently also with small-dimensioned blowers. These conveying devices are what may be referred to as pneumatic conveying troughs. These conveying troughs, also designated as air slides, have a porous bottom, through which blower air or gas is supplied. The pulverulent or fine-granular bulk material is thereby transformed into a flowable state along the conveying zone. In order to transport the bulk material thus fluidized, the pneumatic conveying trough must be arranged at an inclination. The driving force which acts in this case is gravitational force, so that the fluidized bulk material flows downward, following the inclination. The gas requirement of the pneumatic conveying troughs is insignificant, since the gas is used merely for fluidization, not for driving. This has, however, the serious disadvantage that the bulk material cannot be conveyed in any desired direction, but only ever downward.
Various attempts have been made to combine the advantages of the different types of pneumatic conveying devices with one another. Thus, DE-A-11 50 320 discloses a conveying device, in which a pneumatic conveying trough has been extended in the upper space of the conveying trough in order to accommodate an additional suction system. As a result, a vacuum directed toward the end of the conveying trough is generated, thereby assisting a movement of the bulk material along the conveying trough. It is thereby possible that this conveying trough can not only convey in the downward direction, but also at the same level. However, so that this advantageous action can be established, a defined vacuum distribution must be maintained along the conveying trough. For this purpose, it is necessary to subdivide the supplies for the fluidizing gas into individual chambers, and these chambers may not be of equal size, but are smaller at the start of the conveying zone and larger toward the end of the conveying zone. In addition, an inlet valve, through which air can be sucked out of the atmosphere into the conveying trough, is arranged at the start of the conveying zone. This conveying device has a disadvantage that the entire conveying trough has to be provided with fluidizing gas supplies which also have to be designed differently, depending on their position along the conveying trough. Furthermore, the conveying capacity of the conveying trough is very low, since no active propellant gas stream is used, but, instead, there is merely suction via a vacuum. Moreover, vacuum suction often entails the disadvantage that bulk material can easily be sucked into the vacuum system and may lead to wear or defects there. Similar, but simpler devices are described in DE-A-35 35 093 and U.S. Pat. No. 3,056,632. In these, the conveying pipe is provided along its entire length with gas supply means at its lower space. Compressed air is blown into the conveying trough via these means, the pressure conditions being set such that the pressure is highest at the start of the conveying line and gradually falls further along the conveying line. This pressure gradient gives rise to a gas movement in the direction of the conveying trough, with the result that the bulk material is transported in this direction. These devices, too, have the disadvantage that they are complicated, since they require injection means along the entire length of the conveying pipe. A variant of these conveying devices is disclosed in U.S. Pat. No. 2,874,999, in which, however, the supply of fluidizing gas is provided solely in the region of the bulk material feed means and not along the actual conveying pipe.
A further pneumatic conveying device is described in EP-A-0 987 202. In this device, the conveying pipe is provided along its entire length with fluidizing gas supplies on its underside. These are connected to a fluidizing gas source via an overflow valve. The overflow valves are designed in such a way that they close during normal operation and open only as slug is formed in the region of the respective fluidizing gas supply, due to the pressure differences caused as a result of this, serves to allow fluidizing gas to flow in and so as thereby to dissolve the slug. A continuous fluidization of the bulk material to simplify conveyance is not provided.
A further conveying device is known from U.S. Pat. No. 2,919,159. In this, the conveying pipe is not provided with fluidizing gas means along its entire length, but only partially, specifically only at those points where the conveying pipe runs at an upward inclination. This is intended to prevent formation of slugs at these points. Fluidization is therefore provided only partially, to be precise at portions of the inclined section which are particularly at risk. Transport otherwise takes place by means of propellant gas. In order to achieve a sufficient transport capacity in this case, the propellant gas must have a considerable minimum gas velocity. This entails a considerable outlay in terms of the construction and operation of the propellant gas arrangement.